Complexing agents for alkaline earth metal ions and heavy metal ions are used in wide sectors of the industry, for example in the detergents and cleaners industry or in the treatment of metal surfaces. Usually, they are synthesized in aqueous solution. For certain applications, they are required in solid form.
Customary processes for the preparation of solids from aqueous solutions are in particular crystallization and spray-drying processes. It is known that crystalline solid as is produced, for example, during evaporation crystallization or cooling crystallization can comprise water of crystallization and, under ambient conditions, is in most cases less hygroscopic or storage-stable than amorphous solid. As a result of spray-drying processes, e.g. in the spray tower or in the spray fluidized-bed, by contrast, the solid is obtained in amorphous form. In this form, the solid is often highly hygroscopic and, in the event of open storage loses, within a short time, the ability to be poured, which considerably hinders the further processability, e.g. in tabletting presses, etc.
Methylglycinediacetic acid, referred to hereinbelow in abbreviated form as MGDA, also known as α-alaninediacetic acid (α-ADA), is a strong, readily biodegradable complexing agent for various technical applications and is known, e.g. from WO-A 94/29421.
In the form of the racemic trisodium salt mixture, the crystallization is severely inhibited by the asymmetric molecular form, which leads to correspondingly lengthy and uneconomical mass crystallization processes (evaporation crystallization and cooling crystallization).
EP-A 0 8 45 456 describes a process for producing MGDA-Na3 powders with an increased degree of crystallinity, in which the starting materials are in particular starting masses with water fractions of 10-30%, and preferably crystallization seeds are added. This process leads to predominantly crystalline powders, but, on account of the viscous and pasty phases during production, requires the use of complex and expensive mixer-kneader apparatuses in order to ensure conversion to the crystalline modifications.
Accordingly, it was an object of the invention to provide a technically simple process for the preparation of crystalline MGDA trialkali metal salts.
The solution consists in a process for the preparation of a crystalline L-MGDA trialkali metal salt by crystallization from an aqueous solution thereof which has been obtained by Strecker synthesis, starting from L-α-alanine, by reaction with formaldehyde and hydrocyanic acid to give L-α-alanine-N,N-diacetonitrile and subsequent alkaline saponification of the L-α-alanine-N,N-diacetonitrile to give the L-MGDA trialkali metal salt, wherein a temperature of 150° C. is not exceeded during the alkaline saponification.
Surprisingly, it has been found that the crystallization of L-MGDA trialkali metal salts proceeds with considerable advantages compared to the corresponding racemate, with corresponding applications-related advantages of the crystallizate.